masking : Java Glossary

The process of taking apart an int or long into subfields of bits, or clearing the
bits of such a subfield. Strictly speaking, masking just
refers to stripping off excess bits by doing a logical AND with a mask that contains
ones where you want to keep data and zeros where you want it cleared off.

You need skill with the various bit operators to pack several fields into a single
int and then unpack them again. You do this work with >>>, <<, &, | and ~. Rarely you might use the signed shift operator >>. For example to extract the low order three bits you mask
with binary 00000111, e.g. z = x & 0x07. To extract
bits 4 and 5 you shift and mask, e.g. z = x >>>4 & 0x03. To put together a 2-bit x field in bits 4 and
5 and a 3-bit y field in bits 0, 1, 2 you use code like this: z =
x << 4 | y. To zero out a the x field you would take the mask for bits 4
and 5, binary 110000 and invert it and then mask with that, e.g. z &= ~0x30;

Simple Masking

Shift Masking

Combining Fields

To pack subfields into an int
or long, you first get them

// To put together a 2-bit x field in bits 4 and 5// and a 3-bit y field in low order bits 0, 1, 2 you use code like this:long z = (x << 3 )| y;

Creating Masks Dynamically

You can create powers of two, (one-bit masks) by
shifting 1 left a variable amount, e.g. (1<<n).
Normally you create masks with hex literals, e.g. 0x007f.
To dynamically create a mask with n low order 1’s, use (1<<n)-1. Beware, for int this will not work for n=32 since
shifts are done modulo 32.